Role of support in CO2 reforming of CH4 over a Ni/γ-Al2O3 catalyst

A catalyst of 10% Ni/γ-Al₂O₃ for CO₂/CH₄ reforming was prepared and characterized by TPR, TPD, XPS, XRD and activity measurements. XPS and TPR showed that Ni mainly exists in the form of NiAl₂O₄ in the calcined catalyst and is hard to reduce below 650°C, indicating a strong interaction between metal and support. Reduction of the calcined catalyst results in fine particles of Ni⁰, with an average diameter of about 20 nm as determined by XRD. The uptake of H on the reduced catalyst measured by H₂-TPD is 4.2–4.6 mole per mole of Ni species and does not depend on the reduction degree of Ni species. This provides a convincing piece of evidence for the occurrence of hydrogen spillover in the reduced catalyst. Only reduced catalysts present good activity, but the degree of nickel reduction has almost no effect on the reforming activity. This seems to suggest that Ni⁰ is vital for the reforming activity, but γ-Al₂O₃ is also involved in CO₂/CH₄ reforming and contributes even more. Based on the mechanism proposed by Bradford et al. and on our observations, a mechanistic model has been proposed to elucidate the role of γ-Al₂O₃ in CO₂/CH₄ reforming.     

A novel Ni–Mg–Al-LDHs/γ-Al2O3 Catalyst Prepared by in-situ synthesis method for CO2 reforming of CH4

A novel Ni–Mg–Al catalyst derived from layered double hydroxides (LDHs) which was prepared on γ-Al₂O₃ by in-situ synthesis method. The catalyst was evaluated by CO₂ reforming of CH₄ and a better catalytic performance was obtained compared with a reference catalyst of Ni/MgO/γ-Al₂O₃ prepared by impregnation. The novel catalyst was also characterized by XRD, N₂-adsorption-stripping, TEM, TG and AAS (atomic absorption spectrum). The results showed that the excellent performance of the catalyst benefited from its larger specific surface area and smaller active-crystal grain which is due to the molecular-order dispersion of active components over the LDHs precursor.     

CO2 reforming of CH4 over Ni/Mg–Al oxide catalysts prepared by solid phase crystallization method from Mg–Al hydrotalcite-like precursors

Ni supported catalysts were prepared by the solid phase crystallization (spc) method starting from hydrotalcite (HT) anionic clay based on [Mg₆Al₂(OH)₁₆CO₃ ^2–]H₂O as the precursor. The precursors were prepared by the co-precipitation method from nitrates of the metal components, and then thermally decomposed, in situ reduced to form Ni supported catalysts (spc-Ni/Mg–Al) and used for the CO₂ reforming of CH₄ to synthesis gas. Ni²⁺ can well replace the Mg²⁺ site in the hydrotalcite, resulting in the formation of highly dispersed Ni metal particles on spc-Ni/Mg–Al. The spc-catalyst thus prepared showed higher activity than those prepared by the conventional impregnation (imp) method such as Ni/-Al₂O₃ and Ni/MgO. When Ni was supported by impregnation of Mg–Al mixed oxide prepared from Mg–Al HT, the activity of imp-Ni/Mg–Al thus prepared was not so low as those of Ni/-Al₂O₃ and Ni/MgO but close to that of spc-Ni/Mg–Al. The relatively high activity of imp-Ni/Mg–Al may be due to the regeneration of the Mg–Al HT phase from the mixed oxide during the preparation, resulting in an occurring of the incorporation of Ni²⁺ in the Mg²⁺ site in the HT as seen in the spc-method. Such an effect may give rise to the formation of highly dispersed Ni metal species and afford high activity on the imp-Ni/Mg–Al.     

Sol–Gel-Generated La2NiO4 for CH4/CO2 Reforming

A stable La₂NiO₄ catalyst active in CH₄/CO₂ reforming has been prepared by a sol–gel method. The catalyst was characterized by techniques such as XRD, BET, TPR and TG/DTG. The results show that the conversions of CH₄ and CO₂ in CH₄/CO₂ reforming over this catalyst are significantly higher than those over a Ni/La₂O₃ catalyst prepared by wet impregnation and those over a La₂NiO₄/-Al₂O₃ catalyst. The TG/DTG outcome confirmed that the amount of carbon deposition observed in the former case was less than that observed in the latter two cases, a phenomenon attributable to the uniform dispersion of nanoscale Ni particles in the sol–gel-generated La₂NiO₄ catalyst.     

Characterisation of carbon deposits on Ni/SiO2 in the reforming of CH4–CO2 using fixed- and fluidised-bed reactors

CO₂ reforming of methane was investigated with regard to carbon deposition on 4.5 wt% NiO/SiO₂ catalyst at 1023 K, 1 atm and a CH₄/CO₂ ratio of 1.0 employing micro-fluidised- and fixed-bed reactors. A higher catalytic activity (by 20%) was observed in the initial stage (0.5 h) of the fluidised-bed reforming which may be attributed to lesser deactivation of the catalyst compared to fixed-bed operation. Only a limited amount of carbon was deposited in a period of 11 h on stream. In the case of the fixed-bed reactor, a much larger amount of carbon was found on the spent catalyst, particularly, when sampled from the bottom of the bed. TPO results suggest that carbon deposits on the catalyst samples from the fluidised-bed as well as the top of the fixed-bed are rather small and of similar nature. The carbon deposited at the bottom of the fixed-bed reactor contained two distinct species according to XPS results (corresponding to C–O and C–C bonds).     

Synthesis and physicochemical characterizations of Ni/Al2O3–ZrO2 nanocatalyst prepared via impregnation method and treated with non-thermal plasma for CO2 reforming of CH4

Ni/Al₂O₃ and Ni/Al₂O₃–ZrO₂ nanocatalysts synthesized via impregnation and treated with non-thermal plasma were investigated in dry reforming of methane. The results showed that plasma treatment produces highly dispersed nanoparticles with a high surface area. Strong interaction between active phase and support particles in plasma-treated catalysts can be concluded based on XRD and XPS results. Smaller Ni particles with narrow particle size distribution were observed in plasma-treated Ni/Al₂O₃–ZrO₂ nanocatalyst. The catalytic activity of plasma-treated Ni/Al₂O₃–ZrO₂ was higher than that of conventional catalyst, resulting in operating conditions with considerably lower temperatures. Long reaction times confirmed the stability of the plasma-treated Ni/Al₂O₃–ZrO₂ nanocatalyst.     

Dehydrogenation of ethylbenzene with CO2 over V2O5/Al2O3–ZrO2 catalyst

V-containing catalysts supported on Al₂O₃, modified with varying amounts of ZrO₂, were prepared by impregnation method. Dehydrogenation of ethylbenzene with CO₂ was run over these catalysts in a fixed-bed downflow stainless steel reactor. Compared with pure Al₂O₃ support, a small amount of ZrO₂ in the support led to a significant increase in catalytic activity. Partial reduction of vanadium oxides and carbon deposition were the main reasons for the decreased catalytic activity.     

Catalytic CO2 reforming of methane over Ir/Ce0.9Gd0.1O2−x

CO₂ reforming of methane over Ir loaded Ce_0.9Gd_0.1O_2−x (Ir/CGO) has been studied between 600 and 800 °C and for CH₄/CO₂ ratios between 2 and 0.66 in order to evaluate its potential use as an anode material for direct conversion of biogas at moderate temperatures in solid oxide fuel cells. The catalyst exhibited a superior catalytic activity compared to the support alone and other Ir based catalysts. High CH₄/CO₂ ratios and temperatures were required to obtain the maximum H₂/CO ratio, which could never exceed unity. Long-term experiments were carried out, showing the excellent stability of the catalyst with time on stream. Carbon formation was totally inhibited (in most experimental conditions) or very limited in the most severe conditions of the study (800 °C, CH₄/CO₂ = 2). This carbon was found to be highly reactive towards O₂ upon TPO experiments.     

Steam reforming of liquid petroleum gas over Mn-promoted Ni/γ-Al2O3 catalysts

Three different Mn-promoted Ni/γ-Al₂O₃ catalysts, Mn/Ni/γ-Al₂O₃, Mn-Ni/γ-Al₂O₃ and Ni/Mn/γ-Al₂O₃, were prepared and applied to the steam reforming of liquid petroleum gas (LPG) mainly composed of propane and butane. For comparison, Ni/γ-Al₂O₃ catalysts containing different amount of Ni were also examined. In the case of the Ni/γ-Al₂O₃ catalysts, 4.1 wt% Ni/γ-Al₂O₃ showed the stable catalytic activity with the least amount of coke formation. Among the various Mn-promoted Ni/γ-Al₂O₃ catalysts, Mn/Ni/γ-Al₂O₃ showed the stable catalytic activity with the least amount of coke formation. It also exhibited a similar H₂ formation rate compared with Ni/γ-Al₂O₃. Several characterization techniques—N₂ adsorption/desorption, X-ray diffraction (XRD), CO chemisorptions, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and CHNS analysis—were employed to characterize the catalysts. The catalytic activity increased with increasing amount of chemisorbed CO for the Mn-promoted Ni/γ-Al₂O₃ catalysts. The highest proportion of Mn⁴⁺ species was observed for the most stable catalyst.     

CH4 reforming with CO2 for syngas production over nickel catalysts supported on mesoporous nanostructured γ-Al2O3

Nanostructured γ-Al₂O₃ with high surface area and mesoporous structure was synthesized by sol-gel method and employed as catalyst support for nickel catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by XRD, BET, TPR, TPH, SEM and TPO techniques. The BET analysis showed a high surface area of 204m²g^?1 and a narrow pore-size distribution centered at a diameter of 5.5 nm for catalyst support. The results revealed that an increase in nickel loading from 5 to 15 wt% decreased the surface area of catalyst from 182 to 160 m²g^?1. In addition, the catalytic results showed an increase in methane conversion with increase in nickel content. TPO analysis revealed that the coke deposition increased with increasing in nickel loading, and the catalyst with 15 wt% of nickel showed the highest degree of carbon formation. SEM and TPH analyses confirmed the formation of whisker type carbon over the spent catalysts. Increasing CO₂/CH₄ ratio increased the methane conversion. The BET analysis of spent catalysts indicated that the mesoporous structure of catalysts still remained after reaction.     

Performance of Ni/MgO–AN catalyst in high pressure CO2 reforming of methane

The catalytic activity of 8.8 wt Ni/MgO–AN prepared from alcogel derived MgO was studied for the dry reforming of methane under high pressure (1.5 MPa). The catalyst showed a self-stabilization process during the reaction that lasted for 50 h, in which the catalytic activity decreased with increasing the reaction time on stream (TOS) up to 12 h, and then became stabilized thereafter. The activity decline during the initial 12 h of the reaction was found closely related to an increase in the amount of carbon deposits on the catalyst, which also became stabilized after the catalyst had served the reaction for 12 h. Comprehensive characterizations of the coked catalyst with Temprature programmed hydrogenation (TPH), X-ray photoelectron spectroscopy (XPS) and X-ray diffractometer (XRD) techniques revealed two kinds of carbon deposits (-carbon and -carbon) on the used catalyst. The -carbon deposits were found to be produced from CH4 decomposition while the -carbon deposits from CO disproportionation. It was revealed that the accumulation of -carbon deposits was a key cause for the activity decline and the self-stabilized catalysis during the initial 12 h of the high-pressure reaction. Moreover, it was also observed that an unavoidable sintering of metallic Ni particles from 6.5 to 11 nm, which happened within the very first hour of the reaction, was not directly related to the catalyst stability.     

Steam reforming of dimethyl ether over Cu–Ni/γ-Al2O3 bi-functional catalyst：Effect of calcination temperature

采用沉积-沉淀法制备了一系列不同焙烧温度的二甲醚水蒸气重整制氢催化剂2Cu-1Ni/5g-Al₂O₃（摩尔比）,考察了焙烧温度对催化剂2Cu-1Ni/5g-Al₂O₃的结构及催化性能的影响,并运用N₂吸附-脱附（BET）、H₂程序升温还原（H₂-TPR）、X射线衍射（XRD）等手段对催化剂进行了表征与分析。结果表明,500 ℃焙烧的催化剂BET比表面积及孔容、孔径适中。随着焙烧温度的升高,以尖晶石态存在的铜组分比例逐渐增加,金属Cu的粒径也从12.6 nm增至33.2 nm。适当的焙烧温度可保证金属和载体之间的强度适中的作用力,从而保证催化剂具有较优的活性和稳定性。催化剂活性随着焙烧温度的增加先升高后减小,较优的焙烧温度为500 ℃。     

Physicochemical Characterization of Al2O3 Supported Ni–Rh Systems and their Catalytic Performance in CH4/CO2 Reforming

Activity and stability of alumina-supported monometallic Ni, Rh and bimetallic Ni–Rh catalysts were studied towards carbon dioxide reforming of methane. The catalysts were prepared by the incipient wetness impregnation method with different contents of Rh and Ni and were characterized by H₂ chemisorption, TPR_H2, XRD, FT-IR and ToF-SIMS methods. The process of Ni–Rh alloy formation and nickel enrichment on alloy surface takes place during temperature-programmed hydrogen assisted decomposition of their precursors. Catalytic stability and resistance towards coke deposition for Rh/Al₂O₃ and Ni–Rh/Al₂O₃ catalysts are much higher than for Ni/Al₂O₃ and Ni–Rh/SiO₂ systems, studied in the first part of this paper (Jó?wiak WK, Nowosielska M, Rynkowski JM, Appl. Catal. A 280:233, 2005).     

Syngas production via dry reforming of methane over Ni/Al2O3–MgO nanocatalyst synthesized using ultrasound energy

The Ni/Al₂O₃–MgO nanocatalyst with Al/Mg ratio of 1.5 was prepared successfully using sonochemistry method and shown high activity and stability in dry reforming of methane. XRD, BET, FESEM, TEM and EDAX-dot mapping techniques have been used for nanocatalyst characterization. XRD analysis confirmed the formation of MgO and NiO cubic phases. According to the FESEM micrographs, nanostructure grains with uniform surface size distribution have been observed in of the synthesized nanocatalyst. The TEM micrographs showed that ultrasound-assisted preparation method induced uniform morphology without agglomeration of particles. The activity of synthesized nanocatalyst could reach thermodynamic equilibrium conversions and H₂/CO ratios.     

Study on CO2 reforming of methane to syngas over Al2O3–ZrO2 supported Ni catalysts prepared via a direct sol–gel process

Ni-based catalysts supported on Al₂O₃–ZrO₂ (Ni/Al₂O₃–ZrO₂) were prepared by a direct sol–gel process with citric acid as the gelling agent. The evaluation of the catalyst prepared for methane reforming with CO₂ was carried out with thermal gravimetric analysis (TGA), infrared spectroscopy (IR), X-ray diffraction (XRD), microscopy analyses (SEM and TEM), temperature-programmed reduction (TPR) and in a micro-reactor system. The catalytic performance for CO₂ reforming of methane to synthesis gas in a continuous-flow micro-reactor under atmospheric pressure was investigated. TGA, IR, XRD and microscopy analyses show that the Ni particles have a nanostructure of around $5\mathrm{nm}$ and are uniformly dispersed on the Al₂O₃–ZrO₂ support, which exists as an amorphous phase. Catalytic tests using CO₂ reforming of methane to synthesis gas show that the catalytic activity increases with increasing metal loading, and the 20Ni/Al₂O₃–ZrO₂ (0.2 Ni/Al molar ratio) catalyst has excellent activity and stability, compared with that of the Al₂O₃ supported Ni catalyst, with 91.9% conversion of CO₂ and 82.9% conversion of CH₄ over $50\mathrm{h}$ at $1073\mathrm{K}$, atmospheric pressure, hourly space velocity of $11,200\mathrm{ml}{\mathrm{gcat}}^{-1}{\mathrm{h}}^{-1}$ and CH₄:CO₂:N₂ of 2:2:1. The excellent catalytic activity and stability is attributed to the very highly and uniformly dispersed small metallic Ni particles, the reducibility of the Ni oxides and an interaction between metallic Ni particles and the support Al₂O₃–ZrO₂.     

CO2 reforming of methane over coke-resistant Ni–Co/Si3N4 catalyst prepared via reactions between silicon nitride and metal halides

A series of Ni–Co/Si₃N₄ catalysts with different Ni/Co ratios were prepared via reactions between commercial silicon nitride (Si₃N₄) and metal halides (i.e., NiCl₂ and CoF₃) at high temperature (930 °C). By using X-ray diffraction and electron microscopy, it was shown that this method of catalyst preparation leads to formation of bimetallic Ni–Co nanoparticles encapsulated by a SiN_x layer (Ni–Co@SiN_x) on supporting Si₃N₄ material. The 4.0Ni–3.6Co/Si₃N₄ catalyst was highlighted by showing highly stable catalysis for stoichiometric CO₂ reforming of methane under widely varied reaction conditions, and was found completely free of coke formation after CRM reaction for 100 h.     

NO reduction by CH4over La2O3: temperature‐programmed reaction and in situ DRIFTS studies

The chemistry between NO _x species adsorbed on La₂O₃ and CH₄ was probed by temperature‐programmed reaction (TPR) as well as in situ DRIFTS. During NO reduction by CH₄ in the presence of O₂, NO ₃ ^- does not appear to activate CH₄, thus either an adsorbed O species or an NO ₂ ^- species is more likely to activate CH₄. In the absence of O₂, a different reaction pathway occurs and NO^- or (N₂O₂)^2- species adsorbed on oxygen vacancy sites seem to be active intermediates, and during NO reduction with CH₄ unidentate NO ₃ ^- , which desorbs at high temperature, behaves as a spectator species and is not directly involved in the catalytic sequence. Because reaction products such as CO₂ or H₂O as well as adsorbed oxygen cannot be effectively removed from the surface at lower temperatures, steady‐state catalytic reactions can only be achieved at temperatures above 800 K, even though formation of N₂ and N₂O from NO was observed at much lower temperature during the TPR experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon Deposition and Catalytic Deactivation during CO2 Reforming of CH4 over Co/γ-Al2O3 Catalysts

The reaction behavior and carbon deposition during the CO₂/CH₄ reforming reaction have been investigated over the γ-Al₂O₃-supported Co catalysts as a function of Co loading (between 2 and 20 wt%) and calcination temperature (T_c=500 or 1000°C). It was found that the stability of Co/γ-Al₂O₃ catalysts was strongly dependent on the Co loading and calcination temperature. For some loadings (6 wt% for T_c=500°C and 9 wt% for T_c=1000°C), stable activities have been achieved. However, over the catalysts with high Co loadings (>12 wt%), notable amounts of carbon were accumulated during reforming, and deactivation was observed. Moreover, severe deactivation was also noted over the 2 wt% catalysts, both when carbon deposition occurred (T_c=500°C) or was absent (T_c=1000°C). In the latter case, the oxidation of the metallic sites was responsible for the deactivation. Hence, there are two different deactivation mechanisms, namely, carbon deposition and oxidation of metallic sites. The activities were stable when a balance between carbon formation and its oxidation could be achieved.